In recent years, laws and regulations are established for safety driving system of vehicles, such as electronic stability control, collision prevention, and parking assistance system. Further, various applications are being developed and popularized for controlling postures and motions, such as popularization of robots. Accordingly, needs and markets of MEMS (Micro-Electro Mechanical Systems) type acceleration sensors are growing rapidly. In connection with these, needs for measuring the out of plane direction acceleration is increasing. Further, highly reliable and low cost acceleration sensors are required, which have small drifts in sensor characteristic or over time variation (for example, zero point drift or sensitivity variation) even in a location with poor environmental conditions (in terms of temperature, humidity, and vibration) such as engine room of vehicles.
Generally, capacitance detection type acceleration sensor has: a proof mass that displaces corresponding to an applied acceleration; and a sensing electrode that forms capacitance with the proof mass. These components can be made from silicon substrate having multiple layers using photolithographic technology, etching technology, and substrate bonding technology.
In the acceleration sensor described in PTL 1, formed are: a proof mass on a device layer arranged in a plane defined with a first direction and a second direction perpendicular to the first direction; and a support substrate and a cap layer sandwiching the proof mass in a third direction (vertical direction) perpendicular to the first and second directions. The proof mass is plate-shaped and is hung by a support substrate through a torsion beam in a position away from the center of gravity part of the proof mass. Thus, when acceleration is applied in the third direction to the proof mass, the proof mass rotates around the first direction or the second direction. That is, since the rotation center of the proof mass is away from center of gravity part of the proof mass, moment arises in the rotation center in proportion to acceleration applied in the third direction. As a result, the proof mass is displaced toward the third direction.
The displacement of the proof mass in the third direction is detected using two sensing electrodes that are formed in the support substrate side. The sensing electrodes are arranged symmetrically from the rotation center of the proof mass at same distances. Therefore, the proof mass rotates corresponding to acceleration applied in the third direction (z-direction) which is perpendicular to the support substrate plane. In one of the sensing electrodes, which is arranged in a position where the proof mass is approached to the support substrate, the capacitance is increased. On the contrary, in the other sensing electrode, which is arranged in the opposite side symmetrically with the rotation center of the proof mass, and in a position where the proof mass is away from the support substrate, the capacitance is decreased. By detecting the capacitance of these two sensing electrodes with differential detection, electric signals proportional to the acceleration applied in the third direction can be obtained.
The acceleration sensor of PTL 2 below is configured such that a proof mass rotates around a first or second direction, similarly to the acceleration sensor of PTL 1. A sensing electrode is arranged in a cap layer side. In PTL 2, weight is unbalanced by removing a part of the proof mass to realize the rotation of the proof mass and a displacement of the proof mass in the third direction. Therefore, the acceleration sensor of PTL 2 can match the rotation center of the proof mass with center of the cavity, where the cavity is formed by surrounding the proof mass with a support substrate and a cap layer. That is, two sensing electrodes are arranged symmetrically with respect to a geometrical center of the proof mass and the center of the cavity. By arranging the electrodes as such, two sensing electrodes can be displaced uniformly even when distortion arises in an acceleration detection element due to change in circumference temperature, where the sensing electrodes are made of support substrate, proof mass, and a cap layer. Therefore, the capacitance change of the sensing electrode arising from the distortion can be cancelled with the differential detection and can be separated from a signal of an applied acceleration. As a result, an acceleration sensor having small zero point drift, which is caused by mounting or environmental temperature change, can be provided.
In PTL 3 below, a cavity is configured by arranging a support substrate and a cap layer so as to sandwich a proof mass from the upper and lower sides. For purpose of inhibiting deformation of the cavity due to external factors such as environmental temperature change, multiple posts are arranged for connecting the support substrate, a device layer, and the cap layer.
In an acceleration sensor of PTL 4 below, a conductor is formed for penetrating a support substrate or a cap layer as a means for providing an electric signal to a proof mass.